Nanomaterial Properties, Fabrication Techniques & Characterization
Fundamentals of Nanomaterial Properties
At the nanoscale, materials exhibit unique properties compared to their bulk counterparts due to quantum effects and increased surface-to-volume ratio. Here are three examples:
1. Quantum Effects and Energy Levels
Particle dimensions become comparable to electron wavelengths, leading to quantized energy levels. This significantly alters physical and chemical properties.
2. Magnetic Properties (Coercivity)
Coercivity, the resistance to magnetization changes, is affected by particle size. Smaller nanoparticles have higher surface-to-volume ratios, influencing magnetic domains and coercivity.
3. Optical Properties (Color)
Quantum confinement effects cause size-dependent color variations. For example, gold nanoparticles exhibit different colors based on their size due to interactions between electron oscillations (plasmon resonances) and light.
4. Thermal Properties (Specific Heat)
Specific heat, the heat required to raise the temperature of a unit mass by one degree Celsius, is influenced by the higher proportion of surface atoms in nanoparticles. These surface atoms possess different vibrational properties compared to bulk atoms, affecting heat storage and transfer.
5. Electrical Conductivity
While electrons flow freely in bulk metals, at the nanoscale, their motion is constrained, and energy levels become quantized. This can lead to deviations from Ohm’s Law, which assumes continuous energy levels and free electron flow.
Microscopy Techniques: SEM and STM
Scanning Electron Microscope (SEM)
Principle: SEM produces high-resolution surface images using electron-matter interactions. An electron beam replaces the light beam used in traditional microscopy.
Operation:
- An electron gun generates an electron beam focused by specific optics (coils and magnets).
- Electrons collide with the sample, emitting secondary electrons.
- A detector captures these secondary electrons, forming a grayscale image based on their intensity.
- A vacuum environment prevents electron scattering by gas particles.
Advantages:
- Long depth of focus (3D effect)
- Large sample size analysis
- Simple sample preparation
- Wide magnification range
- Simpler column design
Limitations:
- Lower resolution than TEM
- Requires high vacuum
- Conductive samples only
- Surface visualization only (topography)
Transmission Electron Microscopy (TEM)
Principle: TEM illuminates a thin sample with an electron beam in a vacuum, detecting transmitted electrons for morphological, crystallographic, and compositional information.
Operation:
- Electrons pass through a thin sample slice.
- Transmitted primary electrons are analyzed, providing higher resolution than SEM.
- Image contrast reflects electron transmission variations (darker areas indicate less transmission).
Limitations:
- Difficult sample preparation (very thin slices)
- High resolution requires high voltages (expensive)
- Quantification limited to elements with atomic numbers greater than 11
Scanning Tunneling Microscope (STM)
Principle: STM utilizes the quantum tunneling effect, where a particle can pass through a potential barrier. It measures the voltage between a sharp tip and a conductive sample.
Operation:
- A fine tip scans the sample surface, maintained at a close distance.
- Voltage measurements reflect the tunneling current, providing atomic-scale surface information.
- Piezoelectric elements control tip positioning with high precision.
Limitations:
- Sensitivity to vibrations (requires damping mechanisms)
- Requires a sharp, conductive tip
- Tip sharpening (mechanical and chemical) is crucial
Adsorption: Types and Isotherms
Adsorption, a surface phenomenon, involves the adhesion of molecules (adsorbate) to a surface (adsorbent). The specific surface area significantly influences the amount of adsorbate retained.
Types of Adsorption
1. Chemical Adsorption (Chemisorption)
Description: Involves the formation of a strong covalent bond between the adsorbate and the adsorbent surface, often accompanied by a chemical reaction.
Characteristics:
- High specificity
- Site-specific adsorption
- Significant energy changes
- Often irreversible
2. Physical Adsorption (Physisorption)
Description: Involves weaker interactions, such as Van der Waals forces, between the adsorbate and the adsorbent surface.
Characteristics:
- Less specific than chemisorption
- Can occur on any surface area
- Smaller energy changes
- Generally reversible
Common Adsorption Isotherms
Adsorption isotherms describe the relationship between adsorbate concentration and its adsorption onto a surface at a constant temperature. Common isotherms include:
1. Langmuir Isotherm
- Usage: Chemical and physical adsorption
- Principle: Assumes monolayer adsorption on a surface with a finite number of identical sites
- Application: Analyzing reaction mechanisms and surface properties
2. Temkin Isotherm
- Usage: Primarily chemical adsorption
- Principle: Assumes a linear decrease in adsorption heat with coverage due to adsorbent-adsorbate interactions
- Application: Systems with significant adsorbate interactions
3. Freundlich Isotherm
- Usage: Chemical and physical adsorption
- Principle: Empirical model for adsorption on heterogeneous surfaces
- Application: Easy data fitting, used in environmental and chemical engineering
4. BET Isotherm (Brunauer-Emmett-Teller)
- Usage: Physical multilayer adsorption
- Principle: Extends the Langmuir isotherm to multilayer adsorption
- Application: Determining specific surface area of materials
Types of Isotherms:
- Type I: Microporous materials, monolayer adsorption
- Type II: Non-porous materials, multilayer adsorption
- Type III: Porous materials, cohesive forces dominate
- Type IV: Mesoporous materials, staged adsorption
- Type V: Similar to Type III, stronger cohesive forces
- Type VI: Highly uniform surfaces, layer-by-layer adsorption
Nanofabrication Techniques and Examples
1. Chemical Vapor Deposition (CVD)
CVD is a versatile technique for depositing thin films and coatings. It involves chemical reactions of gaseous precursors onto a heated substrate.
Process:
- Transport: Precursor gases are transported to the substrate.
- Adsorption: Precursors adsorb onto the substrate surface.
- Reaction: Chemical reactions occur, forming the desired film.
- Desorption: By-products desorb from the surface.
- Transport: By-products are transported away.
Variants:
- Thermal CVD: High temperatures drive the reactions.
- Plasma-Enhanced CVD (PECVD): Plasma enhances reactions at lower temperatures.
Examples:
- Silicon-based coatings (e.g., silicon dioxide, silicon nitride)
- Carbon nanotubes
- Graphene
2. Physical Vapor Deposition (PVD)
PVD involves the physical transfer of material from a source to a substrate in a vacuum environment.
Process:
- Evaporation/Sputtering: Target material is vaporized or sputtered.
- Transport: Vaporized/sputtered material travels to the substrate.
- Condensation: Material condenses on the substrate, forming a thin film.
Variants:
- Sputtering: Ion bombardment ejects atoms from a target.
- Evaporation: Material is heated until it vaporizes.
- Ion Plating: Combines evaporation with ion bombardment.
Examples:
- Thin film coatings for tools and optical components
- Metallization of electronics
- Decorative coatings
Nanoparticle Characterization
Defining Particle Size
Characterizing nanoparticle size is crucial, but it can be challenging due to variations in shape and aggregation. Techniques like microscopy provide visual information, but quantifying size requires specific approaches.
Common Size Descriptors:
- Equivalent Diameter: Represents the diameter of a sphere with the same property (e.g., volume, surface area) as the particle.
- Specific Surface Area: The surface area per unit mass of a material, often used to estimate particle size assuming a specific shape.
Characterization Techniques
Various techniques are employed to characterize nanoparticle size and distribution:
1. Laser Diffraction
Measures the diffraction pattern of laser light passing through a dispersed nanoparticle sample. The pattern provides information about particle size distribution.
2. Dynamic Light Scattering (DLS)
Analyzes the fluctuations in scattered light intensity due to Brownian motion of nanoparticles in a liquid. Provides information about particle size and diffusion coefficient.
3. Image Analysis
Involves analyzing microscopic images to determine particle size and morphology. Software tools like ImageJ are commonly used.
4. Sedimentation
Measures the settling velocity of nanoparticles in a liquid under gravity. Stokes’ Law is often applied to relate settling velocity to particle size, assuming spherical particles.
5. Sieving
Separates particles based on size using a series of sieves with progressively smaller mesh sizes. Useful for larger nanoparticles.
6. Electrical Sensing Zone (Coulter Technique)
Measures the change in electrical resistance as particles suspended in an electrolyte pass through a small orifice. Suitable for particles in the micrometer range.
Light Scattering Theories
1. Mie Theory
A comprehensive theory that describes the scattering of electromagnetic radiation by spherical particles of any size. It considers factors like particle size, shape, refractive index, and wavelength.
2. Fraunhofer Approximation
A simplification of Mie theory applicable to larger particles where the diffraction pattern can be approximated. Assumes spherical particles and a known refractive index.
Nanoparticle Synthesis Methods
Gas Phase Synthesis
Involves the formation of nanoparticles in the gas phase, often through the reaction or condensation of gaseous precursors.
Examples:
- Inert Gas Condensation: Evaporation of a solid material into an inert gas followed by cooling and condensation to form nanoparticles.
- Pulsed Laser Ablation: A high-energy laser ablates a target material, creating a plume of vapor that condenses into nanoparticles.
- Chemical Vapor Deposition (CVD): Similar to CVD for thin films, but conditions are adjusted to favor nanoparticle formation.
Liquid Phase Synthesis
Involves the formation of nanoparticles in a liquid medium, often through precipitation or controlled growth from solution.
Examples:
- Coprecipitation: Simultaneous precipitation of multiple precursors to form nanoparticles with controlled composition.
- Sol-Gel Process: Hydrolysis and condensation of metal alkoxides in solution to form a gel, which is then dried and calcined to obtain nanoparticles.
- Microemulsion Techniques: Formation of nanoparticles within micelles or reverse micelles in a liquid-liquid emulsion.
Nanofabrication: Top-Down and Bottom-Up Approaches
Top-Down Approaches
Involve starting with a larger material and using techniques to pattern or reduce it to the nanoscale.
Examples:
- Photolithography: Using light to transfer a pattern onto a photosensitive material, followed by etching or deposition steps.
- Electron Beam Lithography: Similar to photolithography, but using a focused beam of electrons for higher resolution.
- Focused Ion Beam (FIB) Milling: Using a focused beam of ions to remove material and create nanoscale structures.
Bottom-Up Approaches
Involve assembling nanoscale building blocks into larger structures.
Examples:
- Self-Assembly: Using molecular interactions to guide the spontaneous organization of molecules or nanoparticles into ordered structures.
- Dip-Pen Nanolithography: Using an atomic force microscope (AFM) tip to deposit molecules onto a surface with nanoscale precision.
- Chemical Vapor Deposition (CVD): Under certain conditions, CVD can be used to grow nanowires or other nanostructures.
Nanofabrication Techniques: Advanced Methods
1. Molecular Beam Epitaxy (MBE)
MBE is a highly controlled technique for growing thin films with atomic precision. It involves the deposition of atoms or molecules onto a heated substrate in an ultra-high vacuum environment.
Applications:
- Semiconductor quantum wells and superlattices
- High-performance lasers and detectors
- Magnetic thin films
2. Focused Ion Beam (FIB)
FIB utilizes a focused beam of ions (typically gallium) to remove material with nanoscale precision. It can be used for milling, deposition, and imaging.
Applications:
- Creating nanoscale structures and devices
- Cross-sectioning and imaging of materials
- Circuit editing and repair
3. Stamp Technology (Nanoimprint Lithography)
Stamp technology involves creating a pattern on a mold or stamp and then transferring that pattern onto a substrate. It is a high-throughput and cost-effective technique for nanoscale patterning.
Applications:
- High-density data storage
- Fabrication of photonic crystals and metamaterials
- Patterning of biomolecules
4. Sol-Gel Process
The sol-gel process is a versatile chemical method for synthesizing materials, including nanoparticles and thin films. It involves the hydrolysis and condensation of metal alkoxides in solution.
Applications:
- Synthesis of ceramic nanoparticles
- Fabrication of porous materials
- Deposition of thin films for optical and protective coatings
Conclusion
s• One unique method it is not useful for all the samples, you have to be really critic with the results obtained.• Particle size distribution depends on the used technique.• You must be critical with the particle size distribution results.• Technique selection is dependent on the nature of the sample and on the particle size distribution characteristic that is more important.
Gas phase synthesis of nanoparticles:
• Solid precursors: Material is vaporized into a background gas and then the gas is cooled.
o Inert gas condensation: A solid is heated to evaporation into a background gas, then vapor is mixed with a cold gas, reducing the temperature. Saturation of the vapour is reached, and nucleation of the nanoparticles occurs. Useful for metals and oxides.
o Pulsed laser ablation: It’s a nice method but it is only useful at lab scale. A pulsed laser is used for evaporating a plume of tightly confined material, generating a small number of nanoparticles. Allows evaporation of materials harder to evaporate.
o Spark discharge generation: Metal electrodes are charged until breakdown voltage is reached and electric arc is formed across the electrodes, vaporizing small amounts of metal.
o Ion sputtering: Vaporization is achieved by sputtering a metal with a beam of inert gas ions at low pressures. Generated nanoparticles can be deposited as nanostructured films on silicon substrates.
• Chemical precursors
o Spray pyrolysis: Widely use method, because you can fabricate tons of nanoparticles. Aerosol process, solution is atomized with a nebulized and injected droplets are evaporated producing solid particles. It is easier to evaporate a small droplet rather than a big droplet. Depending on the temperature, you will obtain different morphologies. You can obtain silica.
o Flame-based synthesis: Gas precursors are injected within a flame. It is the most commercially successful approach for producing carbon black (obtained with an incomplete combustion) and metal oxide nanoparticles. Adjusting flame condition allow a higher control on the size and morphology of the obtained nanoparticles.
Flame spray synthesis: Precursors are sprayed into the flame. Useful for precursors with low vapour pressure (SIO2)
o Chemical vapour synthesis: Vapour-phase precursors are brought into a hot-wall reactor. Conditions favour nucleation of particles in gas phase rather than film deposition. Analogous process to chemical vapour deposition. Doped and multi-component NP can be produced.
o Thermal plasma synthesis: Precursors are injected into a thermal plasma at low pressure, causing atomization. Reaction occurs and particles are formed when cooled. SiC and TiC nanoparticles can be obtained.
o Low temperature reactive synthesis: For certain materials vapour-phase reaction of precursors is possible without external heating. ZnSe NP.
Liquid phase synthesis of nanoparticles
o Coprecipitation: Simultaneous occurrence of nucleation, growth, agglomeration process. Products are soluble species under conditions of supersaturation.
Coprecipitation of metals: Reduction from aqueous solution.
Reduction from non-aqueous solution
• Metal NP prepared by reduction of metallic salts in an organic solvent such as THF, EG, DMF.
Radiation-assisted reduction: Reduction of metal salts in aqueous solution under UV-radiation. Short chain alcohols scavenge UV-generated radicals.
Thermal decomposition of metal-organic precursors: 5-15 nm Fe colloidal nanoparticles can be obtained from Fe(CO)4
Coprecipitation of oxides: Precipitation in aqueous solution, followed by calcination or annealing. When you do the precipitation, the material does not have time nor temperature to form a crystalline structure, it is amorphous, if you want to convert one polymorph (or amorphous) into another, you will have to calcinate it. Lower decomposition temperatures are intended, minimizing agglomeration and aggregation.
Precipitation of oxides from non-aqueous solutions: Useful when two metal hydroxides cannot be simultaneously precipitated in aqueous solution (LiCoO2).
Precipitation of metal chalcogenides by reaction of molecular precursors: Rapid injection of room-temperature solution containing precursors into preheated 350ºC solvent, causing nucleation and growth by controlled heating.
Assisted coprecipitation: Microwave-assisted coprecipitation: MW allow rapid heating. Fast and simultaneous precipitation, better control of particle size and morphology compared to reflux. • Sonification-assisted coprecipitation: Also allows rapid heating by causing cavitation an implosive collapse of bubbles that creates hot spots with high effective
Milling and types of mills Ball mill: The balls used typically are made of ZrO2 (Yttria Stabilized Zirconia). Key points are the size of the spheres, the rotation speed (but until a certain point, because otherwise the balls will attach to the walls and they will not mill, you will know the maximum speed with the noise, at max noise it is the better). The final particle size of ball mill depends on the energy that we introduce, the type of solid and the size of the balls, if you want small particles, you will need small balls. You can have problems with the balls because as they are always receiving hits, some particles of them will detach and will contaminate what is inside the mill.Jet mill Used for ceramic particles, not for soft particles. Vibrating mills You apply vibrations to produce shear stress. When the material is soft, the best approach is to use a cutting mill. Instead of hitting it you will directly cut. Cryogenic milling It is the only way to obtain powders from plastics because it is working in the vitreous transition temperature. Planetary ball mill You can introduce more energy than in a ball mill and it will allow you to crush the materials into smaller sizes. Because as you have 2 rotations, you can overcome this centrifuge limit. Attrition mill
Which techniques are top-down or bottom-up?
Top-down: Breaking down matter into more basic building blocks. Frequently uses chemical or thermal methods. Bottom-up: Building complex system by combining simple atomic-level components.Inert gas condensation Bottom-up Pulsed lased ablation Top-down and bottom-up, depends on the approach. Spark discharge generation Top-down. Milling is top-down but not in the case of mechanochemical, that is bottom-up because you had the reactants and then you obtained the nanoparticles.Nanostructures fabrication:
Nanomanipulation You do atomic manipulation and take atom by atom to create for example a wall.
Photolithography is very expensive, but you can do industrial manufacturing
The mask is not nano, is usually micron size. The chips facilities do not have people because it is all automated and the people only contaminate it.
We take a silicon wafer and deposit a photoresist on it, which is cured through light. We place a mask in between. There are two possibilities for the photoresist:• A positive photoresist• A negative photoresist If we cure the illuminated area, we attack the unprotected areas. A Lithographer’s toolkit:• Oxidation: Oxidation of the wafers• Masking: Masks need to be fabricated (they should not be fabricated through nanofabrication)• Etching: We need to know how to perform etching• Metallization: We need to know how to metallize (deposit thin layers of a metal)• Lift-off: Removal of the resists or layers
Excimer Laser Stepper: Equipment that does not require human intervention. They are used to create chips. UV light of a very low wavelength is used because the shorter it is, the higher the resolution we will have in the fabrication. This light hits the mask. We place a lens in between that will reduce the size of the light beam. We fabricate at the nanometric level because we have the lens in between. E-beam lithography: Used in the lab scale. You need an electron gun that uses a tungsten film to generate electrons. You have the electrons, you focus these electrons (electron optics) and finally you have the substrate (a resin, a polymer). Advantage is does not need a mask and a lent Main difference between E-lithography and photolithography In E-lithography there is no mask, you send the electrons to the place you want to illuminate. Advantage is does not need a mask, but it is very slow. Molecular beam epitaxy You evaporate materials. It is for the fabrication of thin coatings/films. Can cause the removal of the SEM. It is like shooting balines to a wall. Epitaxy: growth of crystalline layers on substrates. We have a chamber. The effusion cells are containers where we place the materials we want to deposit. Now we talk about molecular beams (they act like molecular cannons). This is achieved using the effusion cells. There is an orifice called a molecular leak, which has a diameter smaller than the distance between molecular collisions. This ensures that only molecules traveling in a straight-line exit. In this way, we achieve cannons with straight molecular beams directed towards the silicon wafer. We can deposit multiple layers. With this technique, Quantum well structures can be generated, for example, a Semiconductor superlattice, which are detectors for certain types of radiation (e.g., X-rays). They are designed to detect lower detection limits. Focus ion beam You have ions, and the substrate is heated. Is like shooting cannon balls to a wall. You remove material, it is called milling (fresado). You can remove materials so you can go deeper into a material. You use gallium ions.
CVD (Chemical Vapour Deposition) and PVD (Physical Vapour Deposition) are extremely widely used. Are used for coatings, to protect a surface or depositing a coating to a surface. They are highly used in the fabrication of computers.
CVD: We add some gasses to have a reduction (confirmer). We work in vacuum conditions. WF6 will be used as the limiting reactant. There are chemical reactions, and therefore, we need a certain temperature (usually T>600ºC). At these temperatures, there is a rule of thumb that says thermodynamics predominates. We have enough energy to overcome most kinetic barriers. When thermodynamics predominates, it means that equilibrium is in control. We work with chemical equilibrium reactions.
In a CVD process the parameters that we must control are chemical reactions, thermodynamics, kinetics, transport phenomena and the crystal growth.
Design of CVD Reactors: CVD reactors require specific designs. The layer thickness cannot vary from one wafer to another. All chips must function the same. There are issues that are addressed through design:
a) Horizontal CVD Reactor: The tray is inclined to compensate for the inhomogeneities in the coating of the silicon wafers. We see the cross-section of a coil. It is used to heat the tray by induction, which in turn heats the wafers. An oscillating field generates induced currents. This reactor is not heated resistively, and unlike the previous one, it is a cold-wall reactor (because the heating comes from inside out). This is interesting because less material will stick to the inner walls.
b) Vertical CVD Reactor: We have induction coils. The gas entry is from the top.
Plasma: Ionized gas. It typically emits light because you extract electrons, so the atoms are excited and when they relax, they emit light. You can produce it with an electric field.
Stamp technology: There are several variants: A, B, and C A: Allows the manufacturing of small series. We wet a PDMS stamp (a type of silicone) in a solution that contains molecules ending in a thiol group (R-SH). The thiol groups interact with the gold and bind to it, performing an etching of the sample (any molecule that can be functionalized with a thiol group in the end). We have made conductive tracks of gold that have the shape of the original stamps. This process can be repeated multiple times. Limitation: Most techniques are designed for manufacturing on flat structures. If we have curved structures, we will have problems. Blue spheres: Thiol groups; Sample: Gold (yellow), white and pink
Focused Ion Beam milling (FIB): Nanometric level milling. A milling process removes material. We will remove material with an ion beam that, when it hits our sample, causes material to be removed (right side of the image). The ions in the beam are usually gallium. A constant flow of ions travels down the column. They impact the sample, removing material. As an additional effect, some electrons are emitted, which can be used to create an image like in an electron microscope. All of this is done in a high vacuum. In the FIB, we have gallium ions that remove ions, but also secondary electrons. It also happens that some gallium ions become embedded in the sample. This is not desirable; what we are interested in is being able to fabricate nanostructures.
Sol-Gel process: We start with a precursor that is a metal-organic compound (a compound that contains the metal we want for the nanoparticles). • Solution with a water-alcohol solvent• Formation of the “sol” (colloid) through hydrolysis and condensation• Gelation stage (we form a gel, which is a three-dimensional structure)• Evaporation of the solvent (xerogel or aerogel) to obtain different structures